Hydrodealkylation process and apparatus



Dec. 21, 1965 w, R. LEHRIAN ETAL 3,225,111

HYDRODEALKYLATION PROCESS AND APPARATUS Filed Dec. 19, 1962 2Sheets-Sheet 1 INVEIgORS 1. HR/xi/V 225%? E. NELSON BY VERNON J.YEA/(LEY United States Patent 3,225,111 HYDRODEALKYLATIGN PROCESS ANDAPPARATUS William R. Lehrian, Verona, Elwood E. Nelson, Gibsonia, andVernon J. Yeakley, Oalrmont, Pa., assignors to Gulf Research dzDevelopment Company, Pittsburgh, Pa., a corporation of Delaware FiledDec. 19, 1962, Ser. No. 245,846 4 Claims. (Cl. 260-672) This inventionrelates to a process and apparatus for the hydrodealkylation of alkylaromatics, particularly to the thermal hydrodealkylation of alkylaromatics.

Alkyl aromatics, such as toluene, can be dealkylated to lighteraromatics, such as benzene, by subjecting such alkyl aromatics in thepresence of hydrogen to an elevated temperature and elevated pressurefor a controlled length of time. As a result of such reaction conditionsthe alkyl group is cleaved from the alkyl aromatic and combines with thehydrogen present to form a saturated aliphatic hydrocarbon. The desiredaromatic can be separated from the saturated aliphatic hydrocarbon andunreacted alkyl aromatic and hydrogen, if present, in any convenientmanner.

Thermal hydrodealkylation of alkyl aromatic hydrocarbons, such astoluene, xylenes, trimetl'iylbenzene isomers, alkyl naphthalenes andmixtures thereof, is effected at a temperature in excess of about 1000F., but preferably at a temperature in the range of about 1100 to about1500" F. While the mixture of alkyl aromatic and hydrogen is beingheated to the defined hydrodealkylation temperature little or nohydrodealkylation of the alkyl aromatic occurs. The small amount ofhydrodealkylation which may occur during the preheating stage occurs atthe elevated temperatures at the end thereof. The mixture can bepreheated to the defined hydrodealkylation temperature in the matter ofabout to about 300 seconds.

Once the defined hydrodealkylation temperature is reachedhydrodealkylation occurs swiftly and can be completed in a matter ofabout 10 to about 90 seconds. Since hydrodealkylation of alkyl aromaticsis highly exothermic, at the onset of the reaction there is a tendencyfor the reaction mixture to absorb the heat of reaction and thereby riseto a temperature level in excess of the desired hydrodealkylationtemperature. Near the end of the reaction period, only a small amount ofalkyl aromatic remains, and accordingly only a small amount of heat isliberated when it is dealkylated. This small amount of heat is more thansuificient, however, to maintain the desired hydrodealkylationtemperature level above the desired optimum temperature level. In instances wherein it is desired to maintain a substantially constanttemperature level in the reactor to assure uniformity of product,therefore, the inherent characteristics of the hydrodealkyla-tionreaction make it extremely dithcult to achieve such result.

We have found that the above difficulties can be avoided and thehydrodealkylation of alkyl aromatics can be effected at a selectedtemperature level in an essentially isothermal operation in accordancewith the process and apparatus described and claimed herein. Theadvantages of the present invention can be understood by reference tothe accompanying drawings which form a part of this specification.FIGURE I is an isometric assembly drawing, with parts broken away,showing the construction and design of the preheater and reactor and thehousings therefor. FIGURE II is a graphical representation of dataillustrating the operation of our process using our apparatus therefor.

Referring to FIGURE I there is illustrated a preheater 3,225,111Patented Dec. 21, 1965 furnace 2 and a soaker furnace 4. Preheater 2comprises a housing 6 in which there is mounted an elongated coil 8wherein the mixture of alkyl aromatic hydrocarbon and hydrogen areraised to hydrodealkylation temperature. Soaker furnace 4 comprises ahousing 10 ing which there is mounted an elongated coil 12 wherein thepreheated charge from coil 8 is subjected to the desiredhydrodealkylation temperature.

The charge comprising alkyl aromatic hydrocarbon and hydrogen isintroduced into the system by line 14 adjacent the top of housing 6 at atemperature of about 60 to about 500 F. and a pressure of about 25 toabout 1000 pounds per square inch gauge. Coil 8 as shown is composed ofa series of horizontal sections joined together by appropriate returnbends 16. Mounted on both walls of housing 6 adjacent the horizontalsections of coil 8 are selected numbers of heat-producing elements, suchas gas burners 18, for raising the temperature of the charge in coil 8to dealkylation temperature. In order to facilitate heating of thecharge in coil 8, the consecutive horizontal sections thereof arepreferably not positioned immediately over each other nor in the samehorizontal plane but at an angle from the vertical. Accordingly, as thecharge passes downwardly through the coil the temperature of the chargebecomes progressively higher and as it leaves housing 6 by way of line20 the temperature of the charge is within the range of about 1l00 toabout 1500 F. and the pressure about 25 to about 1000 pounds per squareinch gauge. If desired hydrogen at a temperature of about to about F.and a pressure of about 25 to about 1000 pounds per square inch gaugecan be introduced into coil 8 at one or more points by line 22 and oneor more of lines 24, 26 and 28 in order to help control, by quenchingand dilution, the temperature of the mixture in coil 8 to minimizereaction and avoid or control a runaway reaction. When the mixtureleaves the preheater furnace 2 by way of line 20, little or nodealkylation of alkyl aromatic has occurred. Depending on the severityof the heating and the temperature and pressure of the mixture in coil8, the amount of alkyl aromatic which may be dealkylated in preheaterfurnace 2 may be from about 0 to about 20 percent by weight thereof.Gases resulting from com- 'bustion in burners 18 are removed overheadfrom preheater furnace 2 by stack 30. Although the coils have been shownin FIGURE I in the preferred. embodiment it is apparent that they can bemounted in any desirable fashion within housing 8 and the charge thereincan be made to flow in any desired direction therein provided thetemperature of the charge within the desired time of about 10 to about300 seconds is raised to the defined hydrodealkylation temperature.

The heated charge in line 20, now at a temperature level suitable forhydrodealkylation, is then introduced adjacent the base of housing 10into coil 12 which is positioned and mounted therein in a manner similarto coil 8 in housing 6. Upon entry of the heated mixture into coil 12,therefore, substantial dealkylation of the alkyl aromatic hydrocarbonbegins. Since hydrodealkylation in the present instance is highlyexothermic, the reaction occurs with great liberation of heat. Leftuncontrolled the reaction mixture would have a tendency to absorb theheat of reaction and the temperature thereof would rise beyond thedesired hydrodealkylation temperature level. In accordance with theteachings of this invention the lower portions of the walls of soakerfurnace 4- adjacent the horizontal sections of coil 12, and if desired,the base of housing 10, are provided with openings 32 for permittingcold air to move into housing 10 in direct contact with coil 12. In thisway the cold air absorbs an appreciable amount of the heat of reactionwithin the coil and helps maintain the temperature of the mixture in thecoil at the desired temperature level. In order to control the amount ofair entering housing 10 by way of openings 32, suitable devices, such asdoors 34 pivotally mounted on the exterior of housing 10 can beprovided.

As the mixture rises within coil 12 and hydrodealkylation of alkylaromatic occurs, the amount of alkyl aromatic remaining is progressivelyreduced. The amount of reaction occurring in the upper part of coil 12being small, only a small amount of heat is liberated. Accordingly therate of removal of heat from this portion of the coil can be reduced inorder to maintain the desired temperature level therein. This can bedone by maintaining the temperature level of the air adjacent the coilsat appropriate temperature levels. In accordance with the teaching ofthis invention each of the side walls of housing 10 is provided adjacentthe upper level thereof with selected numbers of heat-producingelements, such as burners 36, for helping to raise the temperature levelof the air in the upper portion of housing 10 to a level sufficient tocontrol the rate of heat removal from the upper portion of coil 12 andthereby maintain the mixture therein at the desired hydrodealkylationtemperature level. Gases resulting from combustion of gases in burners36 are removed from soaker furnace 4 by stack 38. If desired, additionaltemperature control can be obtained in coil 12 by introducing therein atone or more points by line 40 and one or more lines 42, 44 and 46,hydrogen at a temperature of about 80 to about 120 F. and a pressure ofabout 25 to about 1000 pounds per square inch gauge in an amount ofabout to about percent by weight based on the fiow rate of mixture incoil 12.

The temperatures in coil 12 are maintained at a suitable selectedtemperature within the range of about 1100 to about 1500 F. and apressure of about to about 1000 pounds per square inch gauge. A gasspace velocity (actual volume of mixture per hour per volume of reactor)of about 100 to about 300 is maintained within coil 12. The productleaving soaker furnace 4 by line 48 is cooled to a temperature below1000 F. in order to forestall secondary reactions. The product fromsoaker furnace 4 can be cooled in any suitable manner but is desirablycooled to a temperature of about 1000 F. or less by admixture with coldquench from line 50. The cold quench from line 50 can be a hydrogen-richgas, as used for temperature control in line 40, a portion of thedealkylated benzene or naphthalene product or other suitable processmedium, and can be at a temperature of about 60 to about 400 F. Afterthe reactor efiluent is cooled, hydrogen and other gases are vented fromthe reaction mixture and the remainder is separated into its componentparts by any suitable means, preferably by distillation at a temperatureof about 175 to about 250 F. and a pressure of about one to about 10pounds per square inch gauge.

As shown, coil 12, like coil 8 previously described, is composed of aseries of horizontal sections joined together by appropriate returnbends, with the consecutive horizontal sections thereof preferably notbeing positioned immediately over each other nor in the same horizontalplane but at an angle from the vertical. An advantage of thisarrangement is that the horizontal sections are adjacent to the openingsor burners in the walls of the housing 10. In addition the preferreddesign permits the rise of hot air upwardly over the coils in the soakerfurnace 4 and therefore reduces the amount of heat which has to besupplied thereto by burners 36. If desired, however, though notpreferred, the coil in housing 10 could be mounted in a substantiallyhorizontal plane within an appropriate housing with opening in thehousing wall adjacent the first part of the coil and burners adjacentthe latter part of the coil.

It can be seen that the temperature of the air adjacent the lower end ofhousing 10 must be cooler than the air adjacent the upper end thereof,for more heat has to be absorbed in the lower end of the housing than inthe upper portion thereof to maintain substantially isothermalconditions within coil 12. While it is difficult to define specificallythe temperatures which must be maintained at any single point withinhousing 10, in general the temperature of the air therein is maintainedprogressively at temperatures from a low at the base of housing 10 ofabout F. to a high of about 1500 F. adjacent the top thereof, preferablyfrom about 500 to about 1300 F.

In order to withstand the high temperatures involved and to overcomehydrogen embrittlement, the coils in the defined apparatus can be madeof stabilized high alloy austenitic steels such as the following:

Grade Nominal The coils themselves can have a thickness of about A; toabout one inch. The interior of the housings is internally lined with arefractory material.

The invention can further be illustrated by the following. The reaction,similar to that illustrated in FIGURE 1, comprises preheater tubeshaving an inner diameter of 3.75 inches and an outer diameter of 4.50inches and reactor tubes having an inner diameter of 5.625 inches and anouter diameter of 6.625 inches. The total coil length, comprising 20tubes in the preheater and 18 in the soaker furnace, is 760 feet, ofwhich 400 feet are in the preheater section and 360 feet in the soakerfurnace. The total coil volume amounts to 92.81 cubic feet, of which30.68 are in the preheater section and 62.13 are in the soaker section.The coils in the preheater and in the soaker furnace are made of Type347 stainless steel. Toluene at the rate of 20,349 cubic feet per hour,calculated at atmospheric temperature and pressure, and hydrogen at therate of 61,046 cubic feet per hour, calculated at atmospherictemperature and pressure, are introduced into the preheater coils, afterheating by heat exchange means, at a temperature of 350 F. and apressure of 375 pounds per square inch gauge.

The data obtained above have been plotted in FIGURE II. The data do notinclude heat loss from the tube surfaces by radiation and are based onmovement of air around the reactor tubes at the rate of one foot persecond. The data do not take into consideration the sur face or volumeof the return bends of the coils nor of the small amount of line leadingfrom the preheater to the soaker furnace. In the graph the horizontalsection of tubes in the preheater are marked in inverse order, with thefirst horizontal section being considered the 20th tube and the last oroutlet section of tubing being number one. The data in the graph ofFIGURE II show that only about 11 percent conversion has occurred by thetime the preheated charge is introduced into the soaker furnace. Tomaintain the desired temperature level of 1350 F. in the coils adjacentthe entrance thereof requires a rate of cooling of about 7,000 B.t.u.per hour per square foot of coil surface. As the reactants progressthrough the coil and the conversion level increases, the reaction ratedecreases. As the reactor exit is approached, the rate of heatliberation also decreases, and the required rate of cooling falls toabout 300 B.t .u. per hour per square foot of coil. This decreased rateof cooling is maintained by virtue of the air being heated by reactionheat and by the burners located in the upper portion of the soakerfurnace.

Obviously many modifications and variations of the invention, ashereinabove set forth, can be made without departing from the spirit andscope thereof, and therefore only such limitations should be imposed asare indicated in the appended claims.

We claim:

1. Apparatus comprising a first housing and a second housing, eachprovided with a stack therefor, a first elongated coil extending intosaid first housing, disposed therein and extending outwardly thereof,heat-producing elements within said first housing, a second elongatedcoil extending into said second housing, disposed therein and extendingoutwardly thereof, means communicating between said first coil and saidsecond coil, means adjacent the entry of said second coil into saidsecond housing for permitting entrance of air into said second housing,and heat-producing elements in said second housing adjacent the exit endof said second coil.

2. Apparatus comprising a first housing and a second housing, eachprovided with a stack therefor, a first elongated coil extending intosaid first housing, disposed therein and extending outwardly thereof,heat-producing elements within said first housing, a second elongatedcoil extending into said second housing adjacent the base thereof andextending outwardly of said second housing adjacent the top thereof,means communicating between said first coil and said second coil, meansadjacent the base of said second housing for permitting entrance of airinto said second housing and heat-producing elements in said secondhousing adjacent the top thereof.

3. A process for hydrodealkylating an alkyl aromatic which comprisesheating a mixture comprising essentially an alkyl aromatic and hydrogenin an elongated zone to a temperature of about 1100 to 1500 F. andpassing said heated mixture from a lower level to a higher level in anelongated reaction zone composed largely of a series of horizontal runswhile maintaining the temperature of the contents of said reaction zoneat a selected temperature level within a range of about l100 to about1500 F. by passing air having a temperature of about 100 to about 15 00F. progressively upwardly from said lower level to said higher level inindirect contact with the contents of said reaction zone andadditionally heating said air adjacent said upper level.

4. A process for hydrodealkylating toluene which comprises heating amixture comprising essentially toluene and hydrogen in an elongated zoneto a temperature of about 1100 to about 1500 F. and passing said heatedmixture from a lower level to a higher level in an elongated reactionzone composed largely of a series of horizontal runs While maintainingthe temperature of the contents of said reaction zone at a selectedtemperature level within a range of about 1100 to about 1500 F. bypassing air having a temperature of about 100 to about 1500 F.progressively upwardly from said lower level to said higher level inindirect contact with the contents of said reaction zone andadditionally heating said air adjacent said upper level.

References Cited by the Examiner UNITED STATES PATENTS 6/1933 Black eta1. 1961 1O 8/1945 Stewart 260672

1. APPARATUS COMPRISING A FIRST HOUSING AND A SECOND HOUSING, EACHPROVIDED WITH A STACK THEREFOR, A FIRST ELONGATED COIL EXTENDING INTOSAID FIRST HOUSING, DISPOSED THEREIN AND EXTENDING OUTWARDLY THEREOF,HEAT-PRODUCING ELEMENTS WITHIN SAID FIRST HOUSING, A SECOND ELONGATEDCOIL EXTENDING INTO SAID SECOND HOUSING, DISPOSED THEREIN AND EXTENDINGOUTWARDLY THEREOF, MEANS COMMUNICATING BETWEEN SAID FIRST COIL AND SAIDSECOND COIL, MEANS ADJACENT THE ENTRY OF SAID SECOND COIL INTO SAIDSECOND HOUSING FOR